determination server(PDS), and LBS applications in WLAN are presented in this paper. ... Position determination technology on mobile devices such as PDA, ...
The Location-based Services in Local Area using Wireless LAN Seong Ho Lee, Kwang Woo Nam, and Kwang Soo Kim LBS Research Team, Telematics Research Division, ETRI, 161 Gajeong-dong, Yuseong-gu, Daejeon, 305-350, KOREA {sholee, kwnam, enoch}@etri.re.kr
Abstract - The location-based services using location information of mobile devices in wireless LAN are applicable to in-door, hotspot, and downtown areas. The gathering module on signal strength of access points(AP), a positing determination server(PDS), and LBS applications in WLAN are presented in this paper. PDS manages the database including the established location of APs, and determines the position of mobile devices through calculating the signal strength. Moreover, the server provides the applications with GIS maps and location of the mobile device in the form of GML, after the map matching process. Keywords : WLAN, Location Determination, LBS 1. INTRODUCTION Position determination technology on mobile devices such as PDA, notebook and cellular phone is very important for providing location-based services. A number of technologies have been developed to allow determination of user location. Especially, outdoor location sensing technology, such as GPS[1] is already developed and widely used. In other case, we can gather position from a base station’s location of mobile carriers. A new issue is the technology in the busy areas like department stores, downtown and a warehouse in where GPS signals are unreachable. There are a few technologies for location sensing in indoor buildings or on streets among the buildings; for example, GPS pseudolite[2], ultrasonic sensor[3], and cellular-based systems. However, they have some defects. A system of GPS pseudolite has to install pseudo-satellite. A system based on ultrasonic sensor has to install many sensors on a wall, floor or ceiling. In the cellular-based systems, their location accuracy is not satisfactory because the systems provide the location of a base station to user. In this paper, we show the design of position determination system in public space where the IEEE 802.11b wireless LAN(WLAN) network is available. Finally, this system provides the location of mobile devices to client(i.e., viewer) in the form of GML documents. 2. ARCHITECTURE In this paper, we construct a network environment based on wireless LAN infrastructure and support the protocol over the 802.11b. We organize this system by client-server, access points and PDAs as client devices. The gathered position data to be database is provided to client in order to analyze and offer the various services. It is necessary to profile location determination meta data by access points and LAN cards to raise accuracy. Location determination method is implemented on the basis of signal strength, uses the various factors to raise accuracy and triangular surveying. This method utilizes the profile data to be database for correcting the signal strength variation that is very large according to determination environments. 2.1 Infrastructure Network PDA and AP each other connect with 802.11b(Wi-Fi) and the network is composed through connecting AP to location determination server in Ethernet form. A PDA receives radio frequency signals from a neighbor AP, hold BSS IDs, and connect a communication session with AP. We use BSS ID list and RSSI value among attributes supporting in the standard of IEEE 802.11b wireless LAN for determining location.
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Figure 1. Conceptual design of location determination in WLAN 2.2 Location Determination Method In this study, we use access points and a mobile device(i.e., PDA) in wireless LAN environment based on Wi-Fi, the APs are installed on specific location, and location determination method is to measure the distance between APs and PDA. Because signal strength is subject to variation according to atmosphere, the kind of the LAN card, and AP, we should use triangular surveying and signal strength profiles in order to raise the location confidence and accuracy. 2.3 Signal Strength based Location Determination This method is to calculate a distance between the connected AP and device through radio signal strength. We apply ‘Pass Loss Model’ to distance between AP and PDA. Empirical path loss model - indoor
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Figure 2. Empirical path loss model (Wi-Fi and Bluetooth of Microsoft WinHEC) Figure 2 shows the interrelation between signal strength and distance by Pass Loss Model. As shown in Fig.2, the relation changes with different formula at 8m. The relation between signal strength(RSSI) and distance calculated by above model is RSSI = -40.0-20log(d), under 8m, RSSI = -58.5-33log(d/8), over 8m, and the calculation formula for distance is d = 10 ^ (RSSI + 40.0) / -20), under 8 m d = (10 ^ (RSSI + 58.5) / –33) * 8, over 8 m. 2.4 Hybrid Method The relation between RSSI and distance by signal interference has variation. It is the same problem with signal strength based location determination method.
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Figure 3. Interference Between Wi-Fi and Bluetooth Radios Figure 3 shows the relation between Wi-Fi signal strength and distance by Bluetooth signal as interference factor. The signal strength based location determination is subject to variation in terms of radio signal environments. Thus, triangulation or profiles including devices’ features is necessary to correct the above problem. Triangulation can limit an approximate location of a mobile device by finding the intersectional regions for each AP’s signal area. To do this, a mobile device perceives at least 2 AP signals and holds their BSS ID list. 2.5 Data Capture A mobile device captures location determination data and sent it to a server. The server calculates the distance between AP and PDA, and determines the location of PDA with previously mentioned methods. In this paper, the device gathers necessary data on the network structure of Windows CE®. The location determination data includes signal strength, connected BSS ID, signal receivable BSS ID list, and MAC address. We can use WLAN Native SDK that is supplied by WLAN or mobile device manufactories. If not available SDK, we can get the data by accessing network drivers directly using OID that is supported over NDIS 5.1. 2.6 Hierarchical Software Structure In this system, the software is constructed with separated layers by functions and roles. Each layer is only related to upper and lower layers, and supports transparent interfaces for adjacent layers. P r e s e n ta tio n
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Figure 4. Hierachical software structure 2.6.1 Library Layer The library layer consists of interface libraries as DLL and driver implemented by system functions, and libraries is
made up of independent modules by each function. 2.6.2 Wrapper Class Layer This layer contains wrapper classes in order that upper modules use lower modules. It provides transparent access and minimizes the changes of upper module for interface modification caused by the differences among mobile devices, especially WLAN card and its driver software. 2.6.3 Presentation Layer This layer consists of the modules for application execution and user UI. The presentation layer uses the libraries of the lowest layer through the wrapper class, or directly accesses the exported functions by the libraries. 3. EXPRIMENTE AND RESULT 3.1 Signal Strength Profiling In this study, we constructed a database for AP’s signal strength. As stated above, signal strength of the manufactured APs by the various manufacturers vary each other. We made profiles for signal strength for each AP and composed these into matrices. After that, we made the matrices out linear graph for signal strength with MATLAB software. Moreover, the profiles were composed in terms of environment conditions of indoor; concrete block, wood block and line of sight, for example. In general, there are space or points in rooms where radio frequency is unreachable. Thus, we included a parameter such as dead area/points; for example, a pillar.
Figure 5. The measurement result of 3 different cases Figure 5 indicates measurement results that an AP each receives signal strength under the conditions such as concrete block, wood block and line of sight. We can see there is variation for signal strength under the same condition still. 3.2 Experiment Results In this paper, we studied in 20m x 20m square office, used three access points and under the wood block condition. Furthermore, there were three pillars as dead points in this office. We fixed the access points following coordinates; AP1 = (2,3), AP2 = (15,10), AP3 = (3,17), and set up the dead points as (2, 7), (10,7), (18,7).
Figure 6. The original profile graphs for APs and adjusted profile maps Figure 6 shows the original profile graphs received signal strength by the distance and the adjusted profile maps and a environment map. The values in these maps were the Absolute value for the profile value minus current position’s signal strength value of PDA. And, the value for the dead points had the largest value so that be excluded. This PDA received signal strength from three APs each, that is, -75 dBm from AP1, -55 dBm from AP2, and –67 dbm from AP3. Figure 7 tells us that the calculated results and where a PDA is located now. Finally, we know that the location of a PDA is on (13,11).
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Figure 7. The compound map including PDA’s location 3.3 GML for Location Determination In this study, we determined the location of a mobile device in office. After that, the server composed the GML document relevant to that location and sent it a client module. As shown below Fig.8, the location of PDA could be represented with ‘a point’ and the client viewer marked the location.
Figure 8. GML document and client viewer
4. CONCLUSION We designed and implemented the position determination system in indoor space where WLAN network is available, such as office, room, and hotspot zone. In this study, we gathered the profiles for the various access points that those manufactures are different. These profiles are necessary to revise the variation between access points and mobile devices. The server made the GML document with location information for a mobile device, and gave it a client module. Finally, we expect that there will be more application in field of location-based systems if the Wi-Fi based location determination technologies gears the outdoor location determination technologies. REFERENCES [1] U.S. Coast Guard Navigation Center, http://www.navcen.uscg.gov/. [2] Gobb, H.S.: ”GPS Pseudolites: Theory, design, and applications,” Ph.D. Thesis, Stanford University, Sep. 1997. [3] Harter, A., Hopper, A., Steggles, P., Ward, A. and Webster, P.: “The Anatomy of a Context-Aware Application,” Proc. of ACM/IEEE MOBICOM’99, pp. 59-68, Aug. 1999. [4] Kitasuka, T., Nakanishi, T., Fukuda, A., "Wireless LAN based indoor positioning system WiPS and its simulation," Communications, Computers and signal Processing, 2003. PACRIM. 2003 IEEE Pacific Rim Conference on, pp.272 - 275 vol.1, Aug. 2003. [5] Ping Tao, Algis Rudys, Andrew M. Ladd, Dan S. Wallach, "Wireless LAN location-sensing for security applications," WiSE'03, Sep.2003. [6] A. Smailagic and D. Kogan, “Location Sensing and Privacy in a Context-Aware Computing Environment,” IEEE Wireless Communications, vol. 9, no. 5, pp. 10-17, Oct. 2002.
